The present invention relates to a hammer drill having means for guiding a work tool, a power driven percussive mechanism including an electric driving motor coupled via a gear train and a motion convertor to a reciprocating driven member, the driven member acting via an air cushion on an axially movable striking member whose energy is transferred to a work tool, a clutch arranged in the gear train and having a part adjustable into an operative position in response to a contact pressure of the work tool against a workpiece.
A hammer drill of this type is known from U.S. Pat. No. 4,280,359. In this known hammer drill the effectiveness of the releasable clutch between the electric motor and the driven member of the percussive mechanism is dependent on pressure applied by operator on the hammer drill. Apart from the difficult manipulation with the hammer drill, a reliable locking of the driven member of the striker depends on individual conditions inamuch as various operators of the drill cannot apply the same pressure on the latter.